As an optical modulator of an optical transmission system, a Mach-Zehnder type optical modulator using lithium niobate (LiNbO3) or the like (hereinafter, referred to as an “LN optical modulator”) has been known. The LN optical modulator has a good high-speed characteristic and a good chirp characteristic, and therefore is widely used especially for an optical transmission system with a high speed of 10 GHz or higher. A light transmitting/receiving device equipped with the LN optical modulator includes components other than the LN optical modulator; therefore, from the viewpoint of realizing high-density mounting, it is desirable to reduce the size of the LN optical modulator.
In the LN optical modulator, for example, an optical waveguide provided on a substrate and an input/output optical fiber connected to the optical waveguide are arranged so as to extend in the same direction. Therefore, a space for arranging the optical fiber along the extending direction of the optical waveguide is needed, which leads to an increase in the size of the LN optical modulator along the extending direction of the optical waveguide.
Therefore, to prevent an increase in the size of the LN optical modulator, studies have been conducted such that the optical waveguide and the optical fiber are arranged so as to cross each other and an optical path conversion element provided between the optical waveguide and the optical fiber converts an optical path of light such that the optical path goes toward an end portion of the optical waveguide.
Patent Literature 1: Japanese Laid-open Patent Publication No. 5-188238
If the optical path conversion element is provided between the optical waveguide and the optical fiber, for example, the optical path conversion element is fixed on a ferrule that houses an end portion of the optical fiber. Furthermore, the optical path conversion element fixed on the ferrule is inserted from a through path of a housing that houses the substrate on which the optical waveguide is provided, and is fixed on an end surface of the substrate on the optical waveguide side by using an adhesive agent. Moreover, the through path of the housing is sealed by using a solder in order to ensure the airtightness inside the housing.
In this case, if the size of the optical path conversion element is greater than the width of the substrate and the width of the ferrule along the longitudinal direction of the substrate, it becomes difficult to insert the optical path conversion element from the through path of the housing when the optical modulator is constructed. Therefore, a countermeasure such as an increase in the width of the through path of the housing is taken. However, if the width of the through path of the housing is increased, it becomes difficult to seal the through path of the housing with a solder. In this manner, an increase in the width of the through path of the housing may lead to a reduction in the sealing performance of the through path of the housing, which is not preferable.
To cope with this, in some cases, a countermeasure such as a reduction in the size of the optical path conversion element is taken instead of a countermeasure such as an increase in the width of the through path of the housing. For example, it may be possible to reduce the size of the optical path conversion element relative to the width of the substrate and the width of the ferrule along the longitudinal direction of the substrate. If the size of the optical path conversion element is reduced relative to the width of the substrate and the width of the ferrule along the longitudinal direction of the substrate, in some cases, a gap may be generated between the substrate and the ferrule depending on a bonded state of the optical path conversion element and the substrate. If the optical path conversion element is fixed on the end surface of the substrate on an end portion side of the optical waveguide by using an adhesive agent in the state in which the gap exists between the substrate and the ferrule as described, above, the adhesive agent applied to bonded surfaces of the optical path conversion element and the substrate may spill out around the bonded surfaces and may flow into the above-described gap. Consequently, the substrate and the ferrule may accidentally be bonded with the adhesive agent that has flown into the above-described gap. The adhesive agent that has flown into the above-described gap generally has a heat-shrinkable property, and therefore shrinks when temperature of the optical modulator changes. Therefore, with a change in the volume of the adhesive agent caused by thermal shrinkage, the substrate and the ferrule bonded together with the adhesive agent are attracted to each other, and an optical axis of the optical path conversion element fixed on the ferrule is shifted from the initial position. Consequently, a relative positional relationship of the optical path conversion element and the optical waveguide on the substrate is shifted, and an optical loss of the optical modulator may fluctuate.